Such a device is known for instance from U.S. Pat. No. 3,258,166. This document discloses a device comprising a housing with an outlet defining a valve seat, an electromagnetic actuator for discharging metered quantities of liquid from a container, and including a piston which is in sealing engagement with the valve seat when the coil of the electromagnetic device is deenergized, but rises above the valve seat to permit the escape of liquid when the coil is energized. The escape of liquid may be caused by gravity or pressure inside the container. Especially for relatively high viscosity liquids, the liquid may also be forced through the outlet by oscillation of the piston.
The device can function as a valve when the piston due to its reciprocating movement merely opens or closes a fluid path from inlet passage to outlet passage, so that the amount of escaped fluid is, amongst others, determined by the time the fluid path is open. Fluid is then forced out of the container by gravity and/or pressure inside the container and the piston is only reciprocated for opening and closing. This type of dose-measuring device is typically used for low-viscosity fluids.
The device can function as a pump when the piston displaces fluid through the outlet passage using the reciprocating motion. The device, and in particular the piston, may then be provided with valves, in particular non-return valves, such as for instance disclosed in U.S. Pat. No. 4,487,556. Each time, the piston reciprocates, an amount of fluid is dispensed through the outlet. The flow rate is determined amongst others by the stroke of the piston and the frequency of reciprocations. This type of dose-measuring device is typically used for high-viscosity fluids, but can also be used for low-viscosity fluids. As the flow rate is determined by the stroke and frequency of reciprocations of the piston, the amount of dispensed fluid is fairly predictable. This makes this type therefore more suitable for accurate dose-measuring than a device which functions as a valve.
A combination as described in U.S. Pat. No. 3,258,166, in which the device primarily functions as a valve, but oscillation of the piston aids in forcing the fluid through the outlet passage, is also possible.
The piston is actuated by an electromagnetic field interacting with ferromagnetic material in the piston. As is known in the art, an electromagnetic field can be produced in many ways. However, the electromagnetic field is preferably produced with a coil, e.g. solenoid coil or also referred to as electromagnetic coil, as this allows easy control of the electromagnetic field. By appropriately energizing the coil, the piston will move in one direction. The piston may move back to the original position to form a single reciprocation by deenergizing the coil in combination with gravity, pressure inside the container, or a spring force applied by a spring element, or may return to its original position by appropriately energizing the coil again.
As the ferromagnetic material has to interact with an electromagnetic field, the housing is permeable to the electromagnetic field, i.e. permeable to a magnetic flux.
An application of the fluid dose-measuring devices can be found in beverage dispensers using liquid beverage concentrate which is mixed with water, e.g. hot or cold water. The concentrate is then dispensed from a container by a dose-measuring device. Another application can be found in pharmaceutical devices in which for instance two substances have to be mixed, or a single substance has to be delivered to a patient in a specific dose over time.
As known in the art, the dose-measuring device may be permanently integrated with a container which after being emptied is thrown away with preferably minimal influence on the environment. An advantage of combining the container and a dose-measuring device in a disposable unit is that it simplifies the use for an operator or user when installing or removing the container with dose-measuring device as the connection between the container and device is already made and does not have to be removed after usage. The fluid dose-measuring device is therefore also preferably disposable or recyclable. In that case, the solenoid coil is preferably not part of the dose-measuring device, but part of a dispensing apparatus which receives the dose-measuring device with container.
Especially when the dose-measuring device is used for fluids in the food or pharmaceutical industry, the piston may have to meet certain requirements. First, the piston must comprise ferromagnetic material to allow electromagnetic actuation of the piston. Second, the piston is preferably inert to the fluids used, i.e. reactions between piston and fluid must be kept to a minimum or be absent, to prevent contamination of the fluids and damage to the piston, because ferromagnetic materials have very limited corrosion resistance. Third, the piston is preferably fabricated such that it is able to smoothly reciprocate inside the housing, which is advantageous from efficiency and wear point of view.
Current dose-measuring devices try to meet all of these requirements by machining a single piece of ferromagnetic material, e.g. iron, and cover the piston with an protective coating of a material impermeable and inert to the fluid, e.g. electroless nickel coating. To meet the third requirement, the machining and coating is carried out using strict specifications and tolerances. However, as a result, the fabrication process of the piston is complex and time-consuming and provides limited design freedom of the piston itself.